A crankshaft is a principal component of a reciprocating engine, by which power is taken out by converting reciprocating motion of pistons to rotary motion. Generally, there are two types of crankshafts: those that are manufactured by forging and those that are manufactured by casting. For straight-6-cylinder engines for automobiles such as passenger cars, freight cars, and specialized work vehicles, it is necessary that their crankshafts have high strength and stiffness, and therefore forged crankshafts, which are more capable of meeting the need, are widely used. For straight-6-cylinder engines of motorcycles, agricultural machines, marine vessels, and the like, forged crankshafts are also used.
In general, forged crankshafts for straight-6-cylinder engines are manufactured by using, as a starting material, a billet, and subjecting the billet to the steps of preforming, die forging, trimming and coining in order. The billet has a circular or square cross section and has a constant cross-sectional area along the overall length. The preforming step includes roll forming and bending, and the die forging step includes block forging and finish forging.
FIG. 1 is a schematic diagram illustrating a typical conventional process for manufacturing a forged crankshaft for a straight-6-cylinder engine. A crankshaft 1 illustrated in FIG. 1 is to be mounted in a straight-6-cylinder engine. It is a straight-6-cylinder 8-counterweight crankshaft that includes: seven journals J1 to J7; six crank pins P1 to P6; a front part Fr; a flange Fl; and twelve crank arms (hereinafter referred to as “arms” to be simple) A1 to A12 that alternatively connect the journals J1 to J7 and the crank pins P1 to P6 to each other. This crankshaft 1 is a straight-6-cylinder 8-counterweight crankshaft. Among the twelve arms A1 to A12, first and second arms A1 and A2, and the eleventh and twelfth arms A11 and A12 respectively connecting with the first and sixth crank pins P1 and P6 at opposite ends, and fifth to eighth arms A5 to A8 connecting with central third and fourth crank pins P3 and P4 have balance weights. Hereinafter, when the journals J1 to J7, the crank pins P1 to P6, and the arms A1 to A12 are each collectively referred to, a reference character “J” is used for the journals, a reference character “P” for the crank pins, and a reference character “A” for the arms.
According to the manufacturing method shown in FIG. 1, the forged crankshaft 1 is manufactured in the following manner. Firstly, a billet 2 shown in FIG. 1(a), which has been previously cut to a predetermined length, is heated by a heating furnace and then is subjected to roll forming. In the roll forming step, the billet 2 is rolled and reduced in cross section by grooved rolls, for example, to distribute its volume in the longitudinal direction, whereby a rolled blank 103, which is an intermediate material, is formed (see FIG. 1(b)). In the bending step, the rolled blank 103 obtained by the roll forming is partially pressed in a press in a direction perpendicular to the longitudinal direction to distribute its volume, whereby a bent blank 104, which is a secondary intermediate material, is formed (see FIG. 1(c)).
Then, in the block forging step, the bent blank 104 obtained by bending is press forged with a pair of upper and lower dies, whereby a forged blank 105 having a general shape of a crankshaft (forged final product) is formed (see FIG. 1(d)). Then, in the finish forging step, the block forged blank 105 obtained by the block forging is further processed by press forging the block forged blank 105 with a pair of upper and lower dies, whereby a forged blank 106 having a shape in agreement with the shape of the crankshaft is formed (see FIG. 1(e)). In the block forging and the finish forging, excess material flows out as a flash from between the parting surfaces of the dies that oppose each other. Thus, the block forged blank 105 and the finish forged blank 106 have large flashes 105a and 106a, respectively, around the formed shape of the crankshaft.
In the trimming step, the finish forged blank 106 with the flash 106a, obtained by the finish forging, is held by dies from above and below and the flash 106a is trimmed by a cutting die. In this manner, the forged crankshaft 1 is obtained as shown in FIG. 1(f). In the coining step, principal parts of the forged crankshaft 1, from which the flash has been removed (e.g., shaft parts such as the journals J, the crank pins P, the front part Fr, and the flange Fl, and in some cases the arms A), are slightly pressed with dies from above and below and formed into a desired size and shape. Finally, the forged crankshaft 1 is manufactured.
The manufacturing process shown in FIG. 1 is applicable not only to a straight-6-cylinder-8-counterweight crankshaft as exemplified, but also to a straight-6-cylinder-12-counterweight crankshaft (full-counterweight). In a straight-6-cylinder-12-counterweight crankshaft, all of twelve arms A have balance weights. It should be noted that, when adjustment of a placement angle of the crank pins is necessary, a step of twisting is added after the trimming step.
With such a manufacturing method, however, it is inevitable that material utilization decreases since large amounts of unnecessary flash, which is not a part of the end product, are generated. Thus, in the manufacturing of a forged crankshaft, it has been so far an important object to inhibit the generation of flash to the extent possible and achieve improvement of material utilization. Examples of conventional techniques that address this object are as follows.
For example, Japanese Patent Application Publication No. 2008-155275 (Patent Literature 1) and Japanese Patent Application Publication No. 2011-161496 (Patent Literature 2) disclose techniques for manufacturing a crankshaft, by which journals and crank pins are shaped and arms are roughly shaped. In a technique of Patent Literature 1, a stepped round bar having reduced diameter regions at portions to be formed into journals and crank pins of a crankshaft is used as a blank. Then, a pair of the portions to be formed into journals, between which a portion to be formed into a crank pin is disposed are held with dies. In this state, the opposing dies are axially moved toward each other to compressively deform the round bar blank. Concurrently with imparting this deformation, punches are pressed against the portion to be formed into a crank pin in a direction perpendicular to the axial direction to place the portion to be formed into a crank pin into an eccentric position. The above operations are repeated in succession for all crank throws.
Further, in a technique of Patent Literature 2, a simple round bar is used as a blank. Then, one end of the two ends of the round bar is held with a stationary die and the other end thereof is held with a movable die, and portions to be formed into journals are held with journal dies and portions to be formed into crank pins are held with crank pin dies. In this state, the movable die, the journal dies, and the crank pin dies are axially moved toward the stationary die to compressively deform the round bar blank. Concurrently with imparting this deformation, the crank pin dies are moved in an eccentric direction perpendicular to the axial direction to place the portion to be formed into the crank pin into an eccentric position.
With both the techniques of Patent Literatures 1 and 2, no flash will be generated, and therefore a significant improvement in material utilization can be expected.